1. Field of the Invention
This invention relates to systems for controlling work flow in a manufacturing plant and more particularly to dispatching systems therefor.
2. Description of Related Art
Since the product mix and the process technology are complicated in a foundry comprising a semiconductor fabrication plant, different technologies are employed in different stages of the plant. Some of those stages can employ the same types of machines, i.e. equipment, for processing. Because of several factors, WIP (Work In Process) can easily pile up in a particular area on an occasional basis. For example, when one or more of the machines, in one or more of the stages, becomes unstable or where there is a resource arrangement bias that causes the production line to deteriorate into a non-linear status, losses result which comprise reduced wafer movement, increased cycle time, and delayed wafer delivery.
xe2x80x9cBottleneck Starvation Indicators for Shop Floor Controlxe2x80x9d IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING; VOL. 1., No. 1, (FEBRUARY 1988) pages, 147-153.
U.S. Pat. No. 5,396,432 of Saka et al. shows a production system and method of operating thereof.
U.S. Pat. No. 5,751,580 of Chi describes a fuzzy logic method and system for adjustment of priority rating of WIP in a production line.
U.S. Pat. No. 5,768,133 of Chen et al., which is commonly assigned, shows a WIP/Move management tool for a semiconductor manufacturing plant and a method of operation thereof.
U.S. Pat. No. 5,818,716, that is commonly assigned, of Chin et al. describes a dynamic lot dispatching required turn rate factory control system and a method of operation thereof for a semiconductor plant.
U.S. Pat. No. 5,826,238 of Chen et al., which is also commonly assigned, describes a method and a system for a daily target generation and machine allocation with priority.
Characteristics influencing the distribution of WIP are as follows: 1. variable product type; 2. manpower arrangement; 3. machine available time; and 4. dispatching
Indexes that are driven by dispatching of lot of work are as follows: 1. cycle time; 2. OTD (on-time delivery); and 3. wafer movement
1. Using the Lever Principle, the remaining cycle time and consumed cycle time are used to generate the xe2x80x9cDynamically Forward Loading (DFL) Index (IDFL). The Intensity of the DFL Index (IDFL) is referred to as xe2x80x9cxcex1xe2x80x9d. There is a judgement factor used in making decisions pertaining to dispatching which is enhanced by use of xcex1 and IDFL.
According to the xe2x80x9clever principlexe2x80x9d, a lever is in a state of balance when the product of the right side force multiplied by the right side torque arm equals the left side force multiplied by the left side torque arm. Using an analogy to the xe2x80x9clever principlexe2x80x9d and referring to FIG. 5, the elements are as follows:
Fulcrum of lever=Each specific lot/set of lots.
Force applied to right side Wafer quantity of each specific lot/set of lots.
Right side torque arm=Remaining scheduled cycle time counting back from the last stage, i.e. the QC-stage, to the current lot stage.
For example, referring again to FIG. 5, let us assume that there are two (2) lots remaining in production on the line. Those lots are L3 and L4. Let us use a predefined comparison at stage ST2. The fulcrum of the lever separates the right and left sides=L3+L4.
A=Force applied to the right side=wafer quantity of sets of lots L3+L4
B=Right side torque arm=Remaining scheduled cycle time counting from QC-stage to current lots L3+L4
C=Force applied to left side=wafer quantity of sets of lots L3+L4
D=Left side torque arm=Consumed scheduled cycle time counting from stage ST2 to current L3 and L4 stages
Thus, when                     (                  A          xc3x97          B                )                    (                  C          xc3x97          D                )              =    1    ,
the production system is balanced.
2. The IDFL Index immediately indicates any production dispatching bias caused by a factor such as an unstable machine or an inefficient deployment of manpower at each stage which would influence the scheduling and efficiency of the other stages in the manufacturing line.
A computer operated method is employed for determining the rate of dispatching lots for a stage on a production line. First, determine the weighted loading of work for each of the lots between the stage and a succeeding location on the production line by calculating a ratio of weighted progress of the lots as a product of time from dispatching of each lot and as a product of time to completion for each lot to produce an index for the stage. Then, adjust the rate of dispatching of the lots as a function of the Index.
In accordance with this invention the computer system calculates the WIPi for a stage STkk for each lot Li in a queue of lots being processed in a production line between the stage STkk and end point, where xe2x80x9cixe2x80x9d is a positive integer representing the position of the lot Li in the queue, and where xe2x80x9ckkxe2x80x9d is a positive integer indicating the sequential position of the stage STkk (location along the production line) from the beginning to the end of a predetermined portion of the production line. Calculate remaining scheduled cycle time (RCTi) for each lot Li. Calculate consumed scheduled cycle time (CSTi) for each lot Li. Calculate (WIPi*RCTi) for each lot Li. Then, calculate (WIP*CSTi) for each lot Li. Sum WIPi*RCTi for all lots Li of a stage. Sum WIPi*CST for all lots Li of a stage. Calculate the Index of Dynamic Forward Loading Intensity (IDFL) for each stage STkk by dividing the (Sum WIPi*RCTi, for all lots of the stage) by the (Sum WIPi*CSTi, for all lots of the stage) where kk is a positive integer indicating the location of the stage from the beginning to the end of the production line. Adjust the dispatching order of lots based on the rankings of the stages of the production IDFL line, with lowest IDFL ratings having priority.